The present invention relates generally to sub-assemblies used to transfer light between an optoelectronic converter and an optical fiber. More particularly, it relates to optical sub-assemblies (OSAs) comprising a vertical cavity surface emitting laser (VCSEL)
Many different types of optical sub-assemblies suitable for packaging semiconductor lasers have been developed and/or considered by various different groups. In general, these OSAs can be divided into two classes: coaxial OSAs and in-line OSAs. Coaxial OSAs are usually based around a TO (transistor outline) package, which may also be referred to as a TO can. The OSAs containing lasers may also be referred to as transmitter optical sub-assemblies (TOSAs).
Coaxial packages are well suited for vertical cavity surface emitting lasers (VCSELs) since the VCSEL can be mounted directly on a header, with the emission directed out through a window. However, the coaxial, TO type packages typically have poor high-frequency performance characteristics. For example, the high-frequency bandwidth of TO-46 and TO-56 packages may reach a maximum of about 1.5 GHz using typical drive currents and standard mounting configurations. Thus, TO packages are unsuitable for packaging VCSEL sources that are modulated at 10 gigabits per second (Gbps) or higher data rates. Receptacle-type OSAs that use TO packages are similarly limited by the relatively low bandwidth of the TO packages.
In-line packages are typically used for high-speed, edge-emitting laser sources, and typically have a xe2x80x9cpigtailxe2x80x9d of optical fiber that is permanently attached to the laser. The frequency response of these packages typically allows edge-emitting lasers to be driven at 10 Gbps. However, the cost of these packages, which are typically machined from several pieces of Kovar(copyright), can be quite high (typically $20-$30 per package in high volume). Kovar(copyright) is a registered trademark of CRS Holdings, Inc., Wilmington, Del. Further, the basic geometry of the package is designed for edge-emitting lasers. VCSELs can also be used in these packages However, the assembly is typically more complicated than the case of edge emitting lasers, and in-line packages designed for VCSELs may also be relatively expensive.
Most optical sub-assemblies used for coupling the light from an edge-emitting laser diode to a single-mode optical fiber use a lens. The optical fiber can in fact be placed directly in front of the light-emitting facet of an edge-emitting laser diode, however, the coupling efficiency is typically only 10%. This is due to the difference in the mode field diameters of the laser and the optical fiber, and also the astigmatism of the emission from the laser.
The lens is used to convert the size of the laser emission so that it more closely matches the mode field diameter of the optical fiber. Standard xe2x80x9csphericalxe2x80x9d lenses or aspheric lenses are often used for this purpose. The lens adds significant cost to the OSA. Lenses may also require the use of an optical isolator in order to reduce relative intensity noise (RIN) to acceptable levels. Optical isolators typically cost about $100, and tend to significantly increase the cost of OSAs containing them.
Optoelectronic components used in fiber optic communication transmitters and/or receivers are typically required to perform in varying environmental conditions and within tight specifications and tolerances. Thus, when the optoelectronic components are mounted in an OSA, accumulation of moisture within the OSA may have a detrimental effect on such components.
In an exemplary embodiment according to the present invention, an OSA includes a ferrule, an optical fiber disposed within the ferrule, a ceramic or ceramic-like substrate, a seal coupled to the ceramic substrate, and a weld sleeve coupled to the seal and the ferrule. An OSA assembly according to the present invention may advantageously permit a VCSEL mounted on the ceramic substrate to emit light perpendicular to the surface of the ceramic substrate for receipt by an optical fiber.
In another exemplary embodiment according to the present invention, an OSA is provided comprising a ferrule having a substantially cylindrical shape, a weld sleeve, and a microwave-capable ceramic substrate. The weld sleeve comprises a substantially cylindrical member, which is co-axial to and at least partially envelopes the ferrule. The weld sleeve further comprises a flange on one end. The weld sleeve is capable of being laser welded to the ferrule. A seal is mounted on the microwave-capable ceramic substrate, and is capable of being laser welded to the flange.
In still another exemplary embodiment according to the present invention, a transmitter optical subassembly (TOSA) includes a ceramic substrate; a VCSEL mounted on the ceramic substrate, the VCSEL positioned to emit light substantially perpendicular to the surface of the ceramic substrate on which the VCSEL is mounted; a ferrule; and an optical fiber disposed within the ferrule, the ferrule positioned such that an end face of the optical fiber receives light emitted by the VCSEL.
In yet another exemplary embodiment according to the present invention, a method of assembling an OSA is provided. An optical fiber is coarsely aligned with a VCSEL in at least an X-Y direction. The optical fiber is hermetically sealed to a ferrule and the VCSEL is mounted on a ceramic substrate, which has a seal mounted thereon. The distance between the optical fiber and the VCSEL is adjusted in a Z direction until sufficient coupling efficiency is achieved. A weld sleeve is slid over the ferrule to position the weld sleeve on the seal. The weld sleeve is then laser welded to the ferrule, thereby fixing a distance between the optical fiber and the VCSEL in the Z direction. The optical fiber is finely aligned to the VCSEL in at least the X-Y direction. The weld sleeve is then laser welded to the seal.